We report four patients with internuclear ophthalmoplegia and torsiona
l nystagmus, ipsiversive to the side of the medial longitudinal fascic
ulus (MLF) lesion. The MLF lesion could be responsible for an inactiva
tion of the ispilateral interstitial nucleus of Cajal inducing a contr
aversive slow phase. The presence of a corrective ipsiversive quick ph
ase implicates an intact rostral interstitial nucleus of the MLF.